Automatic selective fading control circuit



Jan. 24, 1939. G. L. BEERS 2,144,605

AUTOMATIC SELECTIVE FADING CONTROL CIRCUIT Filed Dec. 18, 1936 2Sheets-Sheet 2 TOAE/VZTWORK i AAAAAAAA ATTORNEY Patented Jan. 24, 1939UNITED STATES PATENT OFFICE AUTOMATIC SELECTIVE FADING CONTROL CIRCUITDelaware Application December 18, 1936, Serial No. 116,500

6 Claims.

My present invention relates to automatic gain control circuits forradio signalling systems, and more particularly to a control circuitwhich is intended to compensate for some of the disagreeable effectswhich are encountered when receiving signals subject to selectivefading.

When utilizing a conventional radio receiver equipped with automaticvolume control to com pensate for normal carrier fading effects,selective fading phenomena produce disagreeable effects in thereproduced signals. One of the most diagreeable effects which selectivefading produces in such a receiver arises from the fact that the audiooutput becomes several times normal. This arises by virtue of the factthat when a carrier wave fades out during the selective fading cycle thepercentage modulation of the received signal is greatly increased, andthe automatic volume control network (hereinafter termed AVC) endeavorsto maintain a constant carrier amplitude at the second detector.

It may be stated, therefore, that it is one of the main objects of mypresent invention to provide a receiver which is equipped with an AVCnetwork, and, in addition, a gain control system which functions toreduce the sensitivity of the receiver when the carrier wave fades outduring a selective fading period thereby preventing the audio outputfrom becoming several times normal.

Another important object of the invention is to provide an automaticvolume control circuit for a radio receiver, and which circuitautomatically reduces the gain of the radio frequency transmission tubesas the received carrier increases in amplitude; and an additional gaincontrol arrangement being provided for reducing the gain of the radiofrequency or audio frequency, transmission tubes when the percentagemodulation of a received signal substantially exceeds 100 per centmodulation whereby the sensitivity of the receiver is automaticallylimited if selective fading causes the percentage modulation of thereceived signal to exceed 100 per cent.

Another object of the invention is to provide a radio receiver with aradio frequency transmission tube and a demodulator, means beingincluded which is responsive to carrier amplitude increase at thedemodulator input for reducing the gain of the radio frequencytransmission tube as long as the percentage modulation of a receivedsignal is less than 100 per cent; additional means: being provided forderiving from the audio output of the demodulator a potential which isemployed to decrease the gain of a radio frequency, or an audiofrequency, transmission tube when the received signal modulation exceeds100 per cent.

Still other objects of the invention are generally to improve theefiiciency of receivers equipped with automatic volume control, and moreespecially to provide an AVC receiver which is not only reliable inoperation, but is economically manufactured and assembled.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawings, in which I have indicateddiagrammatically two circuit organizations whereby my invention may becarried into effect.

In the drawings:

Fig. 1 diagrammatically shows a circuit embodying the invention,

Fig. 2 shows a modification.

Referring now to the accompanying drawings, wherein like referencecharacters in the two figures designate similar circuit elements, thereceiving system shown is of the well known superheterodyne type. Ingeneral, such a receiver comprises a signal collector l which isconstructed to feed signals to the tunable input circuit 2 of the radiofrequency amplifier 3. The numeral 2 denotes the usual variable tuningcondenser for the amplifier input circuit, and it will be understoodthat as many stages of tunable radio amplification as is desired may beprovided. The signal collector I may be of the grounded antenna type,but it may, also, be the usual antenna employed by an automobilereceiver, and it may even comprise a radio frequency distribution line.The amplified radio frequency signals, which signals may be in thebroadcast band of 500 to 1500 k. c. (or in the short wave range when thereceiver is of the all-wave type), are impressed upon the tunable inputcircuit 5 of the converter network. The converter 6 may be of thecomposite local oscillator-first detector type, as, for example, oneusing a GA? pentagrid converter tube.

On the other hand, and if desired, it may employ separate localoscillator and first detector tubes. The numeral 1 denotes the tunablelocal oscillator circuit, and the numerals 5 and 'l' designate thevariable tuning condensers of circuits 5 and 1 respectively. The rotorsof the tuning condensers 2', 5' and I may be arranged in the usualmanner for mechanical uni-control adiustment, and the dotted linedenotes such mechanical uni-control means. Those skilled in the art arefully acquainted with the circuit arrangements used in the converternetwork, and it is believed suflicient for the purposes or thisapplication to point out that in the output circuit 8 there is producedenergy of a desired operating 1. F., and the latter may be chosen from arange of to 500 k. c.

The I. F. energy is impressed upon the input circuit 9, fixedly tuned tothe operating I. F., which functions as the input circuit of the I. F.amplifier H]. The output circuit II of the amplifier I0 is tuned to theoperating I. F., and it is in turn reactively coupled to the I. F. tunedinput circuit |2 of the second detector I3. It is to be understood thatthe amplifier iii may comprise several stages of amplification ifdesired; and while the detector |3 has been shown as of the diode type,any other type of tube may be used. The audio component of the rectifiedI. F. current flowing through load resistor I4, the latter being shuntedby an I. F. bypass condenser, is impressed upon an audio frequencyamplifier l5 through a path including the audio coupling condenser IS.The audio amplifier l5 may coniprise one, or more, stage ofamplification, and the amplifier output is impressed upon an audioutilization network by means of the audio coupling transformer IT. Apart of the audio frequency voltage is impressed upon a diode I8, theanode of the latter being connected to its cathode through a loadresistor I9 which is shunted by bypass condenser 20. It is to beunderstood that the audio voltage for diode l8 may be taken off from theaudio system at any point between the second detector l3 and the finalutilizing device; the latter may be a reproducer of any desired type.

The AVC network of the receiver comprises an electron discharge tube 2|which has its input grid reactively coupled, as through a condenser 22,to the I. F. output circuit of the I. F. amplifier H). The plate of tube2| is connected to the point 23 on the common direct current voltagesupply bleeder resistor P. It is to be understood that the resistor P isthe usual voltage supply resistor, and is fed with direct currentderived from the power supply circuit of the radio receiver. Theconnection to point 23 includes the resistor 25, the cathode of tube 2|being connected to point 26 of bleeder resistor P through a path whichincludes lead 21 and lead 28; the control grid of tube 2| is connectedthrough resistor 50 to point 29 on the voltage supply resistor P. Itwill be observed that the voltage points 23, 26 and 29 are allrespectively of a decreasing positive potential.

The potential difference between points 26 and 29 is such that the tube2| functions as a rectifier of the I. F. energy impressed on the grid oftube 2|, and the direct current voltage developed across resistor 25 isemployed as the AVG bias. The low alternating potential terminals of theradio frequency amplifier, and I. F. amplifier, grid circuits areconnected by connection 33 to the anode side of resistor 25. The numeral3| denotes a filter resistor inserted in the AVG network to suppresspulsating components in the AVG bias. Those skilled in the art know thatthe function of the AVG network is to maintain the carrier amplitude atthe detector input circuit 2 substantially uniform in spite of a widerange of carrier amplitude variation at the signal collector As thereceived carrier amplitude increases the direct current voltagedeveloped across resistor 25 increases, and this voltage is applied in anegative biasing sense to the signal grids of the controlled tubes: withthe result that the gain of each of the latter is reduced therebytending to maintain the carrier amplitude substantially uniform at thedetector input circuit |2.

As pointed out heretofore, one of the most disagreeable effects whichselective fading produces in a receiver of the type described to thispoint, is that due to the audio output becoming several times normal.Since when a carrier wave fades out during the selective fading cycle,the percentage modulation of the received signal is greatly increased,and since the AVG tends to maintain a constant carrier level at thedetector, the audio output is correspondingly increased. This followsfrom the fact that the controlled tubes normally, in the absence ofreceived signals, have a bias such that the tubes are operated withmaximum amplification. Since the AVG bias is derived from the rectifiedsignal carrier, it follows that in selective fading the AVG biasdecreases, and the amplification of the controlled tubes necessarilyrises. It is, of course, understood that in selective fading the carrierfades with respect to its side bands.

To overcome this increase in receiver sensitivity during the selectivefading cycle, the diode I8 is provided for rectification of the audiovoltage produced by the detector I3. The diode anode side of loadresistor I4 is connected to the anode side of resistor l9 through a pathwhich includes an adjustable tap in series with resistor 32. The cathodeside of resistor I9 is connected by a lead, including a resistor 34, tothe control grid of an electron discharge tube 33. The plate of tube 33is connected to the direct current voltage point 23 through the AVGresistor 25, the cathode of tube 33 being connected by lead 28 to point26. The cathode of the second detector I3 is connected by lead 35 to thevoltage point 36 on resistor P, point 36 being at a negative potentialwith respect to point 29.

A condenser 31 is connected between the oathode of tube 33 and the gridside of resistor 34, and a condenser 38 is connected between thejunction point of resistors 32 and I9, and the cathode of tube 33.quency amplifier 3 is connected to the plus B side of the supplyresistor 24, the cathode of the amplifier being connected to a pointwhich is of less positive potential; and the signal grid of theamplifier 3 returns to the still less positive potential point 23 troughthe AVG connection 30 and resistor 25. The tube 33 has been designatedas the selective fading control, since the space current fiow thereof isdirected through resistor 25 for the production of the sensitivityreduction bias when the selective fading occurs.

The direct current potential which is developed across resistor I4 isdirectly proportional to the amplitude of the carrier wave applied toinput circuit i2. If desired, the diode |3 may be a separate diode whichis independent of the second detector. The diode l8 develops a directcurrent potential across resistor l9 which is proportional to the peaksof the audio potentials impressed upon diode l8. The load resistors l4and I9 are connected in series. The tube 33, which may be of the pentodetype if desired, is so connected that its grid potential is determinedby the difference in the direct current potentials derived from thediodes 3 and I8. The tube 33 is provided The plate of the radio frewitha fixed negative bias which is sufiicient to reduce its plate currentsubstantially to the cutoff value when no signals are impressed on thediodes I3 and I8. equal to the potential drop between the points 25 and36 of bleeder resistor P.

The direct current potential developed across resistor I9 causes thegrid of tube 33 to become more positive, whereas the direct currentpotential developed across resistor I 4 causes the grid to become morenegative. This Will be clear when it is realized that the grid of tube33 is connected to the cathode side of resistor I9, whereas it isconnected to the anode side of resistor I4. The circuit constants of thesystem are so adjusted that the direct current potentials developedacross resistors I i and I9 are equal when the percentage modulation ofa received signal is slightly over 100 per cent. If the percentagemodulation exceeds this value then a greater potential is developedacross the output resistor I9 of rectifier I8 than across the loadresistor I4, and the grid of tube 33 will then become more positive.This causes plate current to flow through the resistor 25, since thepositive potential causes the grid of tube 33 to become more positivewith respect to its cathode. Hence, it will be seen that the negativebias on the controlled tubes is. increased, when the direct currentpotential across resistor I9 exceeds that developed across resistor I4.

In this way the sensitivity of the receiver is automatically limitedwhen the percentage modulation of a received signal increases in excessof 100 percent, as. is the case during periods of selective fading. TubeZI is the conventional type of AVG tube, and, as noted before, itfunctions to develop the AVG bias across resistor 25. As long as thepercentage modulation of a received signal remains below 100 per cent,the potential developed across resistor I4 is greater than that obtainedacross resistor I9; and the grid of tube 33 remains at a more negativepotential than that required for plate current cut-off. Under thiscondition the AVG system functions in the normal manner. Therefore, itwill be appreciated that there has been provided in the receiving systemshown in Fig. l a pair of automatic control circuits for the gainregulation of the radio frequency transmission network; one of thecontrols operating to reduce the gain of the radio transmission networkas the signal carrier amplitude increases but the signals have less than100 per cent modulation; and the other gain control system beingresponsive to an increase in the percentage modulation substantiallyabove 100 per cent for the purpose of reducing the gain of the radiofrequency transmission network.

In addition to the beneficial results obtained from the above inventionduring cycles of selective fading, there is also obtained a certaindegree of noise suppression when tuning between stations. This followsfrom the fact that unless a carrier wave is present the potential asdeveloped by the rectifier I8 will be greater than that developed by thediode I3. As a result of this action the sensitivity of the receiverwill be such as to keep the noise output at a relatively low value.

In Fig. 2 there is shown a modification of the invention wherein thetube 2I of Fig. 1 is dispensed with. In place of the tube 2I the seconddetector diode I3 is used to supply the normal AVC bias. The diodeoutput resistor I4 is connected in series with the load resistor 25 ofthe This fixed-cut off bias is selective fading control tube 33. Theresistor 25 is connected to the point 23 on the voltage divider resistorP. The anode end of diode output resistor I9 is connected to the pointon resistor P. The point 40 is sufficiently negative with respect to thepoint 26, to which the cathode of tube 33 is connected, to bias tube 33beyond the point of plate current cut-off by an amount equal to, orslightly greater than, the potential developed across resistor I4 when asignal is being received. It will be observed that the drop across aportion of the voltage divider resistor P is used to supply the negativepotential equivalent to the carrier amplitude, which in the systemillustrated in Fig. 1 was developed across resistor I4 and which must beexceeded by the positive potential developed across resistor I9 beforeplate current is caused to flow through resistor 25.

Since resistors I4 and 25 are connected in series, the AVG bias and theselective fading control bias are both applied to the grids of thecontrolled tubes. It is, of course, clear that the potential developedacross resistor 25 causes the grids of the controlled tubes to assume amore negative potential than that determined by the normal AVC bias.Thus, when the percentage modulation of a received signal exceeds 100%,as

- is the case during selective fading cycles, the

audio output of the receiver will not increase, but can be caused todecrease. The degree of reduction in audio output in either of thearrangements shown in Fig. 1 or Fig, 2 can be varied, for example byutilizing any desired portion of the voltage developed across resistor25.

The potential developed across resistor 25 may be used to control atube, or tubes, other than those controlled by the normal AVC system.For example, in Fig. 2 the anode 42 of tube 33 can be connected to thelow potential end of the grid circuit of an I. F. tube. This. I. F. tubemay be located in the circuit between a point in the I. F amplifier fromwhich the AVG control potential is derived and the second detector. Ifdesired, the contact II may be moved to the point 23, and the AVG andselective fading control systems will then function independently. Theselective fading control potential may also be applied to an audiofrequency amplifier tube. In this case the low potential end of the gridcircuit of the A. F. tube is likewise connected to the anode 42 of tube33. In some cases it is desirable that the controlled A. F. tube beconnected in the audio system after the point to which the diode I8 isconnected. 1

The AVC system used in the arrangement illustrated by Fig. 2 should besuch that the audio output, as determined by the normal AVC will remainsubstantially constant regardless of the strength of the signal beingreceived. If the audio output varies greatly with signal strength, thenthe percentage modulation at which the selective fading control willoperate will vary with the strength of the received signal, since thepotential developed across resistor I9 is compared to a fixed potential,representing the carrier amplitude, thesaid potential being derived fromthe voltage divider. The system illustrated in Fig. 1 automaticallycompensates for any lack of efiectiveness in the AVG, since the voltagedeveloped across resistor I9 is compared to that which appears acrossresistor I4. Thus, if the audio output of the receiver and the voltageacross resistor I9 increases due to any deficiency of the normal AVC,the voltage across resistor I4 must correspondingly increase. Thisrenders the selective fading control system independent of the strengthof the received signal, and dependent only on the percentage modulationof the said signal.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention as set forth in the appendedclaims.

What I claim is:-

1. In a receiving system or the type which includes a modulated carrierwave amplifier followed by a detector, means deriving a direct currentvoltage from the carrier energy and which voltage varies directly withcarrier amplitude, a second means for deriving a direct current voltagefrom the modulation component of the detected signals, a third means forcombining said two voltages in polarity opposition and in such a mannerthat the magnitude of the combined voltage is substantially zero at 100per cent modulation of the modulated-carrier wave, and an additionalmeans, responsive to a departure in po-,

larity and magnitude of said combined voltage from said zero value, forautomatically reducing the gain of said carrier amplifier.

2. In a radio receiver provided with a modulated carrier wave amplifier,a detector including a load resistor for developing a direct currentvoltage from the received carrier current, a rectifier including a loadresistor for developing a direct current voltage from the modulationcurrent of the detected carrier energy, an electron discharge tubehaving an impedance in its space current path, means biasing said tubeto cut-off, means connecting said two'direct current voltages inpolarity opposition and impressing the re-' sultant voltage upon theinput electrodes of said tube, and connections for impressing thevoltage dropacross said impedance upon the amplifier as a gain controlvoltage.

3. Ina radio receiver provided with a modulated carrier wave amplifier,a detector including a load resistor for developing a direct currentvoltage from the received carrier current, a rectifier including a loadresistor for developing a direct current voltage from the modulationcurrent of the detected carrier energy, an electron discharge tubehaving an impedance in its space current path, means connecting said twodirect current voltages in polarity oppositionand impressing theresultant voltage uponthe input electrodes of said tube, connections forimpressing the voltage drop across said impedance upon the amplifier asa gain control voltage, and a second rectifier having a carrier inputconnection to said amplifier output, said impedance being included inthe space current path of said second rectifier,

4. In a radio receiver provided with a modulated carrier wave amplifier,a detector including a load resistor for developing a direct currentvoltage from the received carrier current, a rectifier including a loadresistor for developing a direct current voltage from the modulationcurrent of the detected carrier energy, an electron discharge tubehaving an impedance in its space current path, means connecting said twodirect current voltages in polarity opposition and impressing theresultant voltage upon the input electrodes of said tube, connectionsfor impressing the voltage drop across said impedance upon the amplifieras a gain control voltage, a second rectifier having an input circuitcoupled to the amplifier output circuit, said second rectifier includinga load resistor for developing a direct current voltage from theamplified signals, the voltage drop across the last resistor being inpolarity opposition when applied to said amplifier as regards thevoltage drop across the first resistor.

5. In a receiving system of the type employing s a detector preceded bya radio frequency signal transmission tube and followed by an audiofrequency utilization network, an automatic volume control network forautomatically reducing the transmission efficiency through said radiotransmission network as the received modulated carrier amplitudeincreases, means for overcoming the efiect of selective fading caused byfading of the carrier with respect to its modulation side bands, saidlast means comprising a device for deriving a direct current voltagefrom the carrier energy, a second device for deriving a direct currentvoltage from the modulation current of the detected carrier energy,means combining the last two voltages in polarity opposition, and an additional device, responsive to the resultant of the combined voltages,for automatically reducing the transmission efficiency of the signals tothe detector when the selective fading occurs to an extent such that theeifective percentage modulation of the signals is in excess of 100 percent.

6. In a receiving system of the type employing a detector preceded by aradio frequency signal transmission tube and followed by an audiofrequency utilization network, an automatic volume control network forautomatically reducing the transmission efiiciency through said receiveras the received modulated carrier amplitude increases, means forovercoming the effect of selective fading caused by the fading of thecarrier with respect to its modulation side bands, said last meanscomprising a device for deriving a direct current voltage from themodulation current of the detected carrier energy, means providing asecond direct current voltage which is related in magnitude to a desiredcarrier amplitude, means combining the two direct current voltages inpolarity opposition, and an additional device, responsive to theresultant of the combined voltages, for automatically reducing thetransmission efficiency of the receiver when the selective fading occursto an extent such that the effective percentage modulation of thesignals is excessive.

GEORGE L. BEERS.

